Self-attaching female fasteners, including pierce and clinch nuts, formed by cold header techniques and secondary press operations generally include a central pilot portion having an end face defining a bore. An annular flange portion surrounds the central pilot portion and defines a planar panel-supporting end face having an annular groove in the end face of the flange portion that surrounds the pilot portion. An inner side wall surrounds the pilot portion and is separated from an opposed outer side wall by a bottom wall. Generally, at least one of the inner and outer side walls of the annular groove is inclined toward the other side wall to provide a restricted opening to the annular groove adjacent the end face of the flange portion to improve retention of the fastener following installation in a panel. Further improved retention of the fastener to the panel may be provided by inclining both the inner and outer side walls of the annular groove toward the opposed side walls forming a “dovetail” shaped re-entrant groove. When the panel is deformed against the bottom wall of the groove by a die member or a die button having a projecting annular lip configured to be received in the annular groove, the panel is simultaneously deformed beneath the inclined side wall or walls of the annular groove to provide improved retention of the fastener element to the panel. Various types of anti-rotation or torque resistant elements are disposed in the annular groove defined in the flange portion to provide improved torque resistance when installing a bolt or other male threaded fastener using a torque wrench or the like.
The groove described above, and particularly a groove having inclined outer and inner side walls, provide a substantial amount of pull-off force once the self-attaching fastener has been affixed to a panel. However, a groove of this type has proven less effective for heavy metal panels exceeding about 1.5 mm. Specifically, it has proven difficult to deform the heavy metal panel downwardly into the groove and beneath the undercuts associated with the inclined inner and outer side walls. Therefore, the ability of the inclined outer side wall of the flange portion to provide most of the panel retention of the fastener is known to be reduced drastically. Further difficulties arise when it is necessary to install a self-piercing or clinching fastener into a panel having a restricted area such as, for example, an automotive seat frame or the like. Therefore, it is necessary to reduce the foot print of the fastener without losing either necessary retention or torque resistant characteristics.
It has become evident that there is a heart felt need for self-piercing or clinching fastener which can be used in a heavy metal panel without requiring a large foot print while not sacrificing desirable retention or torque resistant characteristics.